In recent years, what is called a head-mounted type image display device (or a head-mount display) has attracted attention. As for such a head-mounted type image display device, the video image light or the still image light emitted from a compact image display element (e.g. a liquid crystal display panel and an organic electroluminescence) is guided to a user's eye pupil by an optical technique such as a mirror or a prism.
In this case, methods for superimposing the external light and the image light include methods that use an optical element (what is called a combiner) such as a half mirror and a pupil division method. The pupil division method has advantages such as compact, lightweight and a large field of outside view as disclosed in Japanese Patent Application Laid-Open No. 2006-3879, for example.
In addition, an optical system from a image display element to an eye pupil by multiple total reflections of image light inside a prism is known as disclosed in Japanese Patent Application Laid-Open No. 2008-535001, for example.